(i) Field of the Invention
The present invention relates to an electrophotographic photoreceptor having an excellent surface smoothness for use in various printing machines and copying machines.
(ii) Description of the Related Art
Electrophotographic photoreceptors which have now been put to practical use can be classified into an inorganic photoreceptor using an inorganic material and an organic photoreceptor using an organic material. Typical examples of the inorganic photoreceptor include a selenium-based photoreceptor comprising amorphous selenium (a-Se) or amorphous selenium arsenic (a-As.sub.2 Se.sub.3), a photoreceptor obtained by dispersing a dyestuffsensitized zinc oxide (ZnO) or cadmium sulfide (CdS) in a binder resin, and a photoreceptor using amorphous silicon (a-Si).
However, among the above-mentioned inorganic photoreceptors, the selenium-based photoreceptor and the photoreceptor using cadmium sulfide are poor in heat resistance and storage stability. In addition, these photoreceptors contain materials which are harmful to humans, and so they cannot simply be discarded from the viewpoint of an environmental protection and must be collected under regulations. Furthermore, the above-mentioned zinc oxide-dispersed photoreceptor has a poor sensitivity and a low durability, and for this reason, it has scarcely been used nowadays. The above-mentioned amorphous silicon-based photoreceptor has features such as a high sensitivity and a high durability, but owing to its complex manufacturing process, it has a drawback such as the occurrence of image failure.
On the other hand, typical examples of the organic photoreceptor include a photoreceptor using charge transfer complexes of 2,4,7-trinitro-9-fluorenone (TNF) and polyvinylcarbazole (PVK), and double-layered photoconductive structures having a charge generation layer containing a charge generation material for generating a charge carrier at the time of light irradiation and a charge transport layer containing a charge transport material for receiving and transporting the charge carrier generated in the charge generation layer. The organic materials are more present than the inorganic materials, and the organic materials most suitable for the photoreceptor can be selected from a large number of the organic materials, whereby the photoreceptors having the excellent storage stability and no poison can be prepared at a low cost. In recent years, the organic photoreceptors are considered to be most important since durability thereof has been improved.
For the above-mentioned PVK-TNF charge transfer complex-based organic photoreceptor, various improvements have been made, but a sufficient sensitivity has not been attained so far. On the other hand, the above-mentioned function-separated type organic photoreceptors have a relatively excellent sensitivity, and so they occupy most of the organic photoreceptors which have now been put to practical use.
Known examples of such a function-separated type organic photoreceptors include a photoreceptor comprising a charge generation layer formed by applying an organic amine solution of chlorodiane blue and a charge transport layer containing a hydrazone compound (Japanese Patent Publication Sho 55 No. 42380), a photoreceptor comprising a charge generation layer containing a disazo compound and a charge transport layer containing a hydrazone compound (Japanese Patent Application Laid-open Sho 59 No. 214035), and a photoreceptor comprising a charge generation layer containing an azulenium salt compound and a charge transport layer containing a hydrazone compound (Japanese Patent Application Laid-open Sho 59 No. 53850). In addition, there has also been suggested a photoreceptor in which an anthanthrone or a quinone compound which is a kind of pigment is used as a charge generation material (U.S. Pat. No. 3877935).
Each of these organic photoreceptors can be prepared by forming a photoconductive layer on a sheet-like or a drum-like conductive support. As a method of forming the photoconductive layer on the sheet-like conductive support, there is known a method using a Baker applicator or a bar coater. For the sake of the drum-like support, there are known a spray method, a vertical ring method and an immersion coating method, and among these methods, the immersion coating method is usually employed, because a device for use in this method is simple. In these methods, the formation of the photoconductive layer on the conductive support is carried out by applying a coating liquid for the formation of the photoconductive layer onto the conductive support to form a coating film, and then drying this coating film. However, in this formation process, a solvent contained in the coating film is vaporized by the drying, and at this time, an eddy convection is generated in the coating film, so that the surface of the dried coating film becomes uneven and smoothness is lost. This phenomenon is usually called "orange peel".
Heretofore, in order to inhibit the generation of this orange peel, there is known a method in which a solvent having a low vaporization speed is used. However, if this method is particularly used for the drum-like conductive support, the coating film drops during the drying, so that thickness unevenness occurs between top and bottom surfaces of the support. In addition, much time is taken for the drying and productivity also lowers, and for these reasons, the method is not practical. In consequence, this method is unsuitable for the prevention of the orange peel.
In the field of coating materials, a polysiloxane (the so-called silicone oil) is added to a coating material in order to obtain the smoothness on the surface of a coating film. In addition, it is also known that the silicone oil is effective to prevent silking and cratering. Thus, also in the preparation of the electrophotographic photoreceptor, it has been tried to add this silicone oil to the coating solution for the formation of the photoconductive layer, and it has been found that the silicone oil is effective to prevent the orange peel (Japanese Patent Publication Sho 49 No. 15220 and Japanese Patent Application Laid-open Sho 55 No. 140849, Sho 55 No. 5050, Sho 57 No. 212453 and Hei 4 No. 199154). However, these disclosed techniques simultaneously lead to the rise of a residual potential and inconveniently lowers the characteristics of the photosensitive material.
Japanese Patent Application Laid-open Hei 1 No. 234854 discloses an electrophotographic photoreceptor which can be obtained by applying a dispersion comprising a 4,10-dibromoanthanthrone pigment, a polycarbonate and dichloroethane onto a resin intermediate layer to form a charge generation layer, and then further applying there-onto a coating solution comprising the polycarbonate, methylphenylsilicone and 1,2-dichloroethane to form a charge transport layer. The sensitivity of this photoreceptor scarcely deteriorates, but it has a problem that the residual potential largely rises. In consequence, the disclosed photoreceptor is not considered to be satisfactory.
Additionally, in order to obtain the electrophotographic photoreceptor having an excellent surface smoothness without impairing the characteristics of the photoreceptor, it has also be tried that a specific binder resin is combined with a specific polysiloxane to form a photoconductive layer (Japanese Patent Application Laid-open Hei 6 No. 83080 and Hei 6 No. 89038).